1. Technical Field
The present invention relates to a displacement detecting device, a displacement detecting method, and a computer-readable medium which are applied to incremental-type or absolute-type linear or rotary encoder etc. capable of position measurement.
2. Related Art
Photoelectric encoders are known as optical displacement detecting devices which performs a position measurement by reading a pattern on a scale (refer to Patent document 1, for example). The photoelectric encoder of Patent document 1 has a three grating structure in which incident light coming from a light source is input to light-receiving elements via three optical gratings and one of the three optical grating is movable relative to the other two optical gratings.
For example, one of the three optical gratings or, instead, a light-receiving element array is configured in such a manner that its grating pitch or arrangement pitch varies gradually in the relative movement direction according to a pitch variation of a projected interference fringe, that is, the grating pitch or arrangement pitch increases as the distance from the light source increases. With this measure, a light/dark signal corresponding to a scale pattern is prevented from being distorted due to aberrations of an optical system such as a lens and degradation of position information is thus suppressed. Even a portion, far from the light source, of an interference fringe can be handled as a light/dark signal without producing a DC component, whereby the signal detection efficient is increased.